From a meat rendering plant three products are produced, tallow, meal and waste water. The meal will comprise a high proportion of protein and bone, if bone is processed at the same time. This meal has a market as an animal feed the price of which depends on the degree of available protein in the meal. When drying the meal after removal of the tallow and the majority of the waste water, there are two or optionally three requirements. The first requirement is to remove as much water as is required; generally to less than 8%. The second requirement is to avoid denaturing the protein. The third optional requirement is to sterilise the meal when that is necessary. Balancing all three factors together has resulted in drying by indirectly heating through jacketed steam vessels. Temperatures in the vessel in the range of 110.degree.-135.degree. C. are the normal temperatures employed, with residence times of 2-3 hours being used. At these temperatures there is still some denaturing of the protein because of the long residence time. By drying at such temperatures, it is not possible to remove wool or other hair residues. Such may be present in small amounts but such small amounts can lead to a much lower grading for the product and hence a lower price. Sterilisation can be achieved by other means than in the drying step.
Rotary driers have been used heated by combustion of a suitable fuel. Because of fires which have commonly developed, such driers have not proven to be commercially successful for drying organic materials.
The reason for such fires is considered by the current applicant to be that the rate of flow of the material passing through the naked flame was not adequately controlled.
There are other materials where a product needs to be dried to a predetermined moisture content. A number of standards for various materials require drying to achieve a moisture content of 8% or less. At this level of moisture, most products are stable for long term storage. The problems of drying to a predetermined moisture level are the difficulty in establishing precisely when that moisture level has been reached. Thus overdrying is normally undertaken to ensure compliance with the standard. This results in increased fuel cost and increased damage to the product. Other products where moisture content may vary are blood from meat processing works, grain, sewage sludge and the like. The majority of these materials are of organic origin, and it is drying of these materials which is the main purpose of the current invention. It is however envisaged that the process and the apparatus of the invention can be used to dry other materials.